Recently, in the bread manufacturing industry, a method for making bread with refrigerated dough has been widely used with the purpose of saving labor in the bread making process and meeting diverse needs of consumers. In this method, partially fermented dough is stored at a low temperature in a refrigerator and then is subjected to fermentation, proofing and baking to make bread. Such a method is usually carried out by the use of refrigeration-resistant yeast, that is, yeast which is capable of controlling fermentation during the storage of dough at a low temperature and allowing normal fermentation at temperatures for fermentation and proofing to raise the dough.
As for the breeding of refrigeration-resistant yeast, there are known methods in which yeast strains of wild type are conferred with the mutation exhibiting low-temperature-sensitive fermentability by artificial mutagenesis [e.g., Japanese Published Examined Patent Application No. 71474/95, Japanese Published Unexamined Patent Application No. 213277/95, Japanese Published Unexamined Patent Application No. 79767/95, and Appl. Environ. Microbiol., 61, 639-642 (1995)]. The yeast strains conferred with the mutation exhibiting low-temperature-sensitive fermentability are used as refrigeration-resistant yeast or as parent strains for breeding refrigeration-resistant yeast.
However, such mutagenesis induces mutation at random and thus may possibly confer the yeast with mutation relating to the basic properties of fermentation such as dough raising, in addition to the low-temperature-sensitivity mutation.
It is also known to confer baker's yeast or brewer's yeast with favorable properties such as flocculation [The 23rd European Brewery Conv. Proc., 297-304 (1991)] and flavor [Curr. Genet., 20, 453-456 (1991)] by using gene manipulation techniques.
However, a gene relating to the low-temperature-sensitivity of fermentability or a method for breeding refrigeration-resistant yeast by gene manipulation is not known.
Ethanol is produced by fermentation of sugar materials (e.g. molasses) or starch materials (e.g. corn and potato) as carbon sources. Fermentation can be generally carried out at a temperature of 30 to 43.degree. C. Usually, the fermentation temperature is adjusted to 30 to 35.degree. C. by cooling in order to avoid the death, insufficient growth, or decrease in fermentability of yeast caused by the rise of temperature. However, in the summer months, cooling is often insufficient, thereby causing the rise of culturing temperature to 35 to 38.degree. C. in the course of alcohol fermentation. Thus, alcohol fermentation is usually carried out with further cooling to prevent the rise of temperature due to fermentation heat. A need exists for temperature-resistant yeast which is useful for saving cost for cooling in such process.
As for the breeding of thermotolerant yeast, there have been reports on a method in which mitochondria relating to thermotolerance is introduced [Juan Jimenez, et al.: Curr. Genet., 13, 461-469 (1988)] and a method in which heat shock protein HSP104 is expressed at a high level [Susan Lindquist, et al.: Proc. Natl. Acad. Sci. USA, 93, 5301-5306 (1996)]. However, application of these methods to alcohol fermentation has not been studied. Further, it is known that the heat-resistance of yeast is improved by heat treatment at temperatures which are not fatal to the yeast [B. G. Hall: J. Bacteriol., 156, 1363 (1983)], but this effect is not lasting, and it is difficult to apply this method to alcohol fermentation.